The present invention relates to a liquid-development electrophotographic apparatus which uses a high-viscosity, high-concentration liquid toner, and more particularly, to a liquid-development electrophotographic apparatus in which an oil layer of low solid concentration is formed at a surface region of a toner layer on a developing roller before development is performed.
FIG. 4 shows the overall configuration of a conventional liquid-development-type electrophotographic apparatus (as disclosed in, for example, Japanese Patent Application Laid-Open (kokai) No. 11-174852). An illustrated photosensitive drum 10 is electrostatically charged by means of a charger 21. Subsequently, the photosensitive drum 10 is exposed to light by means of an exposure unit 22, whereby an electrostatic latent image is formed. A prewetting unit 23 applies to the surface of the photosensitive drum 10 silicone oil having a viscosity of about 2.5 cSt to a thickness of 4-5 xcexcm.
Developing units 24 corresponding to yellow, magenta, cyan, and black are provided, and use as a liquid developer a nonvolatile, high-viscosity, high-concentration liquid toner having a toner viscosity of 400-4000 mPa-S and a carrier viscosity of 20 cSt. Abutting the photosensitive body, a developing roller supplies the liquid developer onto a photosensitive body so as to maintain on the photosensitive body a two-layer structure composed of the liquid developer and prewetting liquid, and causes toner particles contained in the liquid developer to adhere to the photosensitive body according to an electric field established between the same and the photosensitive body.
Developer applicator means conveys the liquid developer from a toner fountain to a developing roller while spreading the liquid toner thinner, to thereby form on the developing roller a toner layer having a thickness of 2-3 xcexcm. The developer applicator means includes a train of rotative rollers and conveys the liquid developer such that the liquid developer is applied to the surface of the rotative rollers while being spread. A final-stage rotative roller abuts a developing roller to thereby apply a liquid developer film, which is formed on the surface thereof, to the surface of the developing roller.
An intermediate transfer roller 15 is biased at about xe2x88x92800 V and transfers toner from the photosensitive drum 10 sequentially; for example, in the order of yellow, magenta, cyan, and black, according to an electric field established between the same and the photosensitive drum 10. A pressure roller 19 is adapted to fix on a printing medium toner which is melted on the intermediate transfer roller 15 by means of a heating unit 28. The heating unit 28 partially heats the surface of the intermediate transfer roller 15 at a position located upstream of a position where the intermediate transfer roller 15 comes into contact with the pressure roller 19. Reference numeral 26 denotes a blade for scraping off residual toner, and reference numeral 27 denotes a destaticizer.
In order to suppress fogging at a non-image area, which would otherwise occur as a result of viscous adhesion of toner fluid, a developing unit which uses a high-viscosity, high-concentration liquid toner requires a prewetting process for applying an insulating liquid to a photosensitive drum before development is performed.
In a known technique, voltage is applied between an applicator roller and a developing roller in order to apply a high-viscosity toner to a developing roller in the form of a thin toner layer. Also, there has been proposed a technique of applying an electric field to a developing roller in order to create a state in which toner particles have cohered (refer to Japanese Patent Application Laid-Open (kokai) No. 9-62109).
However, when a high-viscosity, high-concentration liquid toner is used, stress induced by viscosity of the toner liquid is very large as compared with an electrostatic force which causes toner particles to move. As a result, at the time of destructive separation at a toner conveyance section and a development section, a layer is disturbed greatly, with a resultant involvement of the following problems: poor levelness of a formed layer, impairment in image uniformity, and tendency of fogging. Application of a bias enables establishment of a state in which toner particles have been collected on the developing roller side at the nip section by means of an electric field. However, at the separation section, which is the exit of the nip section, viscosity of the toner liquid causes disturbance of toner.
Also, application of an electric field of high intensity enables cohesion of toner which can overcome viscosity. However, an excessively high degree of cohesion causes strong adhesion of toner to a developing roller, resulting in impairment in development density or a failure to collect residual toner after development.
An object of the present invention is to obtain a high-quality image in a configuration which uses a high-viscosity, high-concentration liquid toner, by imparting the following features to the configuration: an electric field is applied beforehand to a toner layer on a developing roller to thereby form at a surface region of the toner layer an oil layer of low solid concentration; an unnecessarily high degree of toner cohesion is not caused, and disturbance of the toner layer due to liquid viscosity is suppressed, to thereby form a level, transparent oil layer at a surface region of the toner layer; and the resultant toner layer is brought into contact with an image bearer body to thereby perform development.
Another object of the present invention is to apply a thin toner layer uniformly at a position located upstream of a member for applying an electric field, to thereby perform development effectively.
Still another object of the present invention is to avoid layer disturbance which would otherwise be caused by viscosity of toner at an exit of a nip section where a developing roller and a photosensitive drum effect nipping, to thereby obtain an image of high quality.
A liquid-development electrophotographic apparatus of the present invention comprises a developer bearer body which is in contact with an image bearer body having an electrostatic latent image formed thereon so as to supply a liquid toner serving as a liquid developer onto the image bearer body and which causes toner particles contained in the liquid developer to adhere to the image bearer body according to an electric field established between the same and the image bearer body to thereby form a toner image. The apparatus further comprises means for conveying and applying the liquid toner onto the developer bearer body in the form of a level, thin layer, and a conductive blade which comes into contact with a liquid toner layer on the developer bearer body. A predetermined amount of liquid toner is allowed to pass through between the conductive blade and the developer bearer body. Voltage is applied to the conductive blade so as to bring a surface region of the level, thin toner layer on the developer bearer body to a state such that toner particles contained in the liquid toner are sparsely scattered. The resultant liquid toner, whose surface region is in a state such that toner particles are sparsely scattered, is brought into contact with the image bearer body for performing development.